Synthetic-resin construction panels are known which basically comprise a pair of rigid outer plates sandwiching a mass of hardened synthetic-resin foam between them. Such panels are provided with edge stiffeners which at the same time must offer considerable resistance to bending while having minimal weight and low heat-conducting capacity. It is necessary that the sandwich panel resist forces at right angles to its plane while at the same time it is necessary that such a panel be limitedly expansible and contractile, principally within its plane, to compensate for temperature changes.